1. Field of the Invention
The present invention relates to a pre-pit detector of an optical recording medium capable of writing data, such as a DVD-R.
2. Description of the Related Art
A write-once type DVD (Digital Versatile Disc or Digital Video Disk) has already become practical as an optical recording medium capable of writing data. A rewritable DVD-RW has also been studied for practical use.
FIG. 1 is a perspective view showing the structure of a write surface of a DVD-R or DVD-RW (which will be hereinafter called merely the xe2x80x9cDVDxe2x80x9d and its section. FIG. 1 shows the state before an information read/write apparatus writes information data (audio data, image data and computer data).
Protuberant groove tracks GV (data write track) and recessed land tracks LD (pre-pit tracks) are alternately formed either spirally or concentrically on a write surface of a DVD before information data is written, as shown in FIG. 1.
A plurality of land pre-pits LPP which represent addresses representing each position on the groove track GV and which give the write timing are previously formed on each land track LD. Each of the land pre-pits LPP takes the form of interconnecting adjacent groove tracks GV, and its surface (the surface indicated by oblique lines) is located on the same plane as the surface of the groove tracks GV.
An information write/read apparatus for writing the information data to the DVD, irradiates a write beam corresponding to the information data to the groove track GV while detecting the land pre-pit LPP from the DVD and recognizing the position on the groove track GV. By heat transfer to a region to which the write beam is irradiated, an information pit is formed in the region.
A pre-pit detector for detecting the land pre-pit includes a four-split optical detector 11 as shown in FIG. 2. The four-split optical detector 11 comprises a photo-electric conversion device having light reception surfaces 11a to 11d that are divided into four parts by directions extending along the information write tracks (groove tracks GV) of the DVD and by the directions orthogonal to the write tracks. The light reception surfaces 11a and 11d are positioned on the outer circumferential side of the disk and the light reception surfaces 11b and 11c, on the disk inner circumferential side.
The read beam is irradiated by a read beam generator to the DVD driven for rotation by a spindle motor, forming an information-reading spot on the write surface of the DVD. The photo-electric conversion device receives the beam reflected from the forming region of the information-reading spot on the DVD, on each of the four light reception surfaces 11a to 11d, and outputs light reception signals Ra to Rd as electric signals corresponding to the reception light intensity of the reception light surfaces 11a to 11d. The light reception signals Ra and Rd corresponding to the respective light reception surfaces 11a and 11d positioned on the disk outer circumferential side are applied to an adder 12. The light reception signals Rb and Rc corresponding to the light reception surfaces 11b and 11c on the disk inner circumferential side are applied to an adder 13. The adder 12 adds the light reception signals Ra and Rd and the adder 13 adds the light reception signals Rb and Rc. Furthermore, a subtractor 14 subtracts the output signal of the adder 13 from the output signal of the adder 12, and the output signal of the subtractor 14 is obtained as a radial push-pull signal.
It will be assumed hereby that an information-reading spot by a beam irradiated as shown in FIG. 3 exists at a position that contains a land pre-pit LPP with one groove a track GV located in the center of the information-reading spot on a DVD to which no information data is written at all. In the case, since the reflected light intensity to the light reception surfaces 11a and 11d of the optical detector 11 decreases due to diffraction of the optical beam, and the reflected light intensity to the light reception surfaces 11b and 11c increases, the level of the output signal of the adder 12 decreases to a level lower than that of the output signal of the adder 13. Therefore, the radial push-pull signal issued from the subtractor 14 has a waveform having a sharp valley as shown in FIG. 4 in response to the land pre-pit LPP. The radial push-pull signal is supplied to a digitizing circuit 15 and is digitized at a predetermined threshold value, so that the land pre-pit LPP is detected.
The land pre-pits LPP having the information such as the address of one track are formed on the outer circumferential side of the track. Therefore, each of the land pre-pits LPP on the outer circumferential side is detected as shown in FIGS. 3 and 4.
However, when the pre-pit detector described above detects the land pre-pit LPP from a DVD on which information data is written, and the reflected light intensity at the time of detection of the land pre-pit LPP adjacent to the write mark portion decreases as will be described later. The digitizable range of the radial push-pull signal issued at the time becomes narrow. Further, the information data components of the adjacent tracks leak into the radial push-pull signal, and the information-reading spot deviates from the track center by an error occurred in tracking control for the information-reading spot. For these reasons, the radial push-pull signal fluctuates greatly as if a land pre-pit LPP was detected during the period in which the land pre-pit does not exist as shown in FIG. 5, the digitizable range becomes also narrow. The digitizable range exists between the peak value of the radial push-pull signal corresponding to the land pre-pit LPP and the peak value of the period in which the land pre-pit does not exist, as shown in FIG. 5. As the digitizable range becomes narrower, setting of the threshold value for digitization becomes more difficult, and the land pre-pit LPP cannot be detected correctly.
It is therefore an object of the present invention to provide a pre-pit detector that can expand a digitizable range of a radial push-pull signal irrespective of leak of information data components from adjacent tracks and irrespective of the deviation of the balance due to a tracking error of an information-reading spot, and can correctly detect the pre-pit.
A pre-pit detector for an optical recording medium including a write surface on which an information write track for forming information pits carrying information data and pre-pit track previously having a plurality of pre-pits carrying information associated with the information write track are repeatedly formed in pairs, comprising: optical detection means having light reception surfaces split as first and second light reception surfaces in a tangential direction of the track, for receiving a reflected light of an optical beam irradiated to the write surface on the first and second light reception surfaces so as to output first and second optical detection signals corresponding to light reception intensities of the first and second light reception surfaces, and for receiving the reflected beam from the pre-pit track pairing up with the information write track to which the optical beam is irradiated, on the first light reception surface; subtraction means for computing a difference between the first and second optical detection signals generated from the optical detection means; and digitization means for digitizing the output signal of the subtraction means so as to generate the pre-pit detection signal, wherein the optical detection means has amplification means for giving to the first optical detection signal an amplification result by a greater gain in comparison with the second optical detection signal so as to output the first and second optical detection signals to the subtraction means.
The construction described above can expand the digitizable range of the radial push-pull signal as the output signal of the subtraction means even when the leak of the information data components from the adjacent tracks exists, or when the variation of the balance resulting from the tracking error of the information-reading spot exists. Since the construction makes it easier to set the threshold value for digitization, the pre-pits of the pre-pit track can be detected more correctly.